let box = document.querySelector('.box')

box.addEventListener('mouseenter',function(){
  console.log("进入元素");
  this.classList.add('enter')

  setTimeout(() => {
    this.classList.remove('enter')
  }, 5000);
})

box.addEventListener('mouseleave',function(){
  console.log("离开");
  // this.classList.remove('enter')

})

box.addEventListener('transitionend',function(e){
  console.log(e,"过渡事件结束触发");
})